1. Field of the Invention
This invention relates to flowmeters. More particularly it relates to thermal flowmeters to measure the flow of fluids, liquid or gas, in a conduit. Thermal flowmeters have been known. Many of the preferred or necessary associated elements and techniques are in themselves known. The particular and important contribution of this present invention is the concept of using a pair of radial junction thermocouples for sensing the temperature as part of the flow measuring process.
Previous flowmeters utilized different theories of operation. A great many of them require the insertion of some form of obstruction or impedient into the path of the flowing fluid. The present invention permits the use of an absolutely smooth and unobstructed and unimpeded interior flow path through the conduit. In fact, the flowing fluid "sees" no difference in its pipe or conduit whether it is passing through the flowmeter or not.
In the known development of thermal flowmeters, the use of thermocouples as the sensing elements has been known. However, it is not believed that there has been any prior disclosure of a radial junction thermocouple in the application or environment of the radial junction thermocouple surrounding a conduit through which flow flows.
In addition to the advantage of avoiding obstructions, reductions or impediments in the conduit, a very significant advantage accrues through using a radial junction thermocouple as opposed to a conventional thermocouple or a point thermocouple. Some of the advantages of the present invention's radial junction thermocouple thermal flowmeter include very fast response time, very high sensitivity, very high repeatability, very high turndown ratio, avoidance of interference of the fluid flow characteristics or disturbance of the normal flow, much greater accuracy and stability of readout because of the inherently great temperature sensing averaging function or characteristic, much greater current output because of the much greater length of the dissimilar metal interface.
Other advantages of the present invention include the fact that it is applicable to a very great range of rate of flow, a very great range of characteristics of the flow, that is, its viscosity, density, pressure, temperature and the like, and to a very great range of size of the system in which it is incorporated. That is, this invention has applicability to very small miniaturized systems, such as are found in analytical laboratories or in clinical medical applications, with conduit sizes on the order of magnitude of 1/8" for example. It also has application to very large chemical process and materials handling applications involving conduits having internal diameters measuring in terms of feet or meters.
2. Description of the Prior Art
It is old and common to provide measurement of fluid flow in a conduit by means of two temperature sensors spaced along the conduit, and a heater to provide heat to one of the sensors preferentially when fluid is flowing, and to measure the differential output of the sensors. Such sensors may be thermoelectric. An example is paragraph 4.3.6 in "Handbook of Transducers for Electronic Measuring Systems", Harry N. Norton, Prentice-Hall, 1969. Another is pages 142-144 in "Instrument Engineers Handbook", Liptak and Venczel, Chilton Book Company, 1969, 1982. Another is U.S. Pat. No. 4,255,968, which also discloses one prior art example of the electronic circuitry. U.S. Pat. No. 4,460,802 discloses a radial junction thermowell. A number of other patents disclose thermocouples mounted on conduits, including U.S. Pat. Nos. 3,444,740; 3,874,239; 2,752,411; 2,048,680; 3,901,080; 3,143,439; 3,099,922; 2,607,808; 4,164,433; and 4,447,6587. None of these patents or other references, taken individually or together in their teachings, suggest the radial junction thermocouple flowmeter of the present invention.